1. Field of the Invention
This invention relates to a method and apparatus for sensing the depth of a recess formed in the end of articles, such as headed fasteners.
2. Background Art
It is common in the manufacture of lightweight metal fasteners to produce the fasteners in an automated process, whereby hundreds of articles are formed each minute. In order to maintain a high degree of quality control, it is important to have the capability of examining a high volume stream of fasteners to determine critical dimensions of the articles, such as the depth of recesses formed in the head of the fastener.
Typically, a plurality of fasteners are advanced along a manufacturing line by means of physically engaging the extending flanges of a headed portion in a gripping mechanism and sequentially feeding the fasteners towards storage receptacles. A common way of efficiently feeding a fixed number of fasteners, say 10 or 20, in consecutive batches through an examination station is to load a batch of fasteners into a carriage which is then incrementally indexed through the examination station, whereby each fastener is in turn subjected to examination. The fasteners are supported within the carriage by means of engagement of the head of each fastener with an adapted receiving surface, with a shank portion of the fastener extending below the carriage.
In order to examine the external dimensions of an article, such as thread formation or fastener length, it is known (see Great Britain Patent Specification Nos. 1,604,841 and 1,604,842) to illuminate the fastener as it is supported on the carriage and project a magnified silhouette of the fastener onto an image-receiving sensor. Based upon a photoelectric determination of the dimension of the silhouette, electric signals are relayed to a controller which ultimately result in the examined fastener being either accepted or rejected.
A significant problem with the above examination technique is that the determination of a interior dimension of a fastener, such as the depth or formation of a concave recess formed in the head of the fastener, is precluded. While it is technically possible to measure the recesses formed in the fasteners determined to be `acceptable` by the high speed examination of the external dimensions, such as by a handheld depth gauge, it is extremely inefficient to handle such a high volume of articles twice.
In addition to inefficiency, further problems exist with respect to performing automatic recess examinations. In order to determine the depth of a recess, one may engage a compleementary element, such as a die, with the head of the fastener and measure the amount of penetration of the die into the head to ascertain the acceptability of the recess. However, such a procedure is dependent upon the die being aligned with the contoured surfaces which define the recess in such a way as to allow complete interengagement of the two members. The alignment problem is aggravated by the fact that the fasteners are randomly oriented on the carriage. In the event that the die is misaligned prior to engagement, the distance which it may be inserted into the fastener is limited not by the depth of the recess, but instead by interference with a portion of the head. This may result in a signal which will cause rejection of an acceptable item. In order to obtain a reliable measurement of the depth of a recess automatically, it is imperative that the measuring gauge be capable of complete engagement with each fastener.